Electroluminescent displays (hereinafter, abbreviated as “ELDs”) are light-emitting electronic display devices, which include inorganic electroluminescent elements or organic electroluminescent elements (hereinafter, also referred to as “organic EL elements”). Although the inorganic electroluminescent elements have been used as flat light sources, alternating high voltages are necessary for driving the light-emitting elements.
The organic electroluminescent elements each include a cathode, an anode, and a luminous layer containing a luminous compound disposed between the cathode and the anode. The organic electroluminescent element generates excitons by recombination of electrons and holes injected into the luminous layer and emits light (e.g., fluorescent or phosphorescent light) through deactivation of the excitons. Since the organic electroluminescent element can emit light at a voltage of approximately several to several tens of volts, is of a self-luminescent type to provide a large viewing angle and high visibility, and is a thin-film completely solid element, it has attracted attention from the viewpoint of space saving and portability for example. In order to sufficiently utilize the characteristics of the completely solid element, it has been investigated to produce flexible elements with a flexible plastic sheet or metal foil instead of rigid substrates.
The organic electroluminescent elements are surface light sources, which is also a remarkable characteristic of the organic electroluminescent elements, unlike the major traditional light sources that have been practically used, such as light-emitting diodes and cold-cathode tubes. This characteristic can be effectively used in lighting sources and back lights of a variety of displays. In particular, it can be suitably used in backlights of full-color liquid crystal displays, which have been significantly demanded recently.
In contrast to traditional organic EL elements used in lighting merely emitting light of a single color, organic EL elements that can control color tones and color temperatures of white light can create additional values, and investigation has been being continued (see, for example, PTLs 1 and 2). Unfortunately, the control of color tones by these disclosed methods causes a problem in viewing angle dependence because of an increase in the thickness of the organic layer and the presence of an intermediate electrode having a refractive index different from those of layers on both sides of the electrode. As a technology for dealing with this problem in viewing angle dependence, disclosed is a method for effectively extracting light, emerging from an organic EL element with a smooth particle layer disposed between the substrate and the electrode (e.g., see PTL 3). A method for reducing the viewing angle dependence by adjusting the thickness of each sublayer of a barrier layer composed of SiO2/SiNx/SiO2 on a plastic substrate is disclosed (see, for example, NPL 1). Unfortunately, even these methods cannot provide a sufficiently uniform light distribution, and the problem of viewing angle dependence still remains.
There is a tradeoff between the emission efficiency and the drive life. A proposed countermeasure to solve such a relation is a toning-type organic electroluminescent element including a laminate of a plurality of luminous units separated by intermediate layers, each unit being a light emission functional layer including a luminous layer made of an organic material. This element has a long lifetime while securing an emission efficiency. For example, PTL 4 discloses a structure including an electrically conductive layer, in a floating state, mainly composed of magnesium (Mg) and containing silver (Ag). PTL 5 discloses a structure including a laminate intermediate electrode composed of a metal layer having a low work function, such as Mg, Mg/Ag, or arsenic (As), and an indium tin oxide (SnO2—In2O3, hereinafter abbreviated as “ITO”) layer. PTL 6 discloses a structure including a transparent conductive layer of, for example, ITO as an intermediate electrode and an electroluminescent layer including a sublayer made of a material resistive to etching, such as a benzoxazole derivative or a pyridine derivative, where the sublayer is closest to the intermediate electrode.
These methods, however, cannot provide an emission efficiency and a service life that are sufficient for practical use.